Spectrophotometers are employed to measure diffuse reflectance in many narrow spectral bands, and these devices are employed, for example, in the color measurement of such substances as paints, plastics, textiles, and the analysis of powdered chemicals or biological specimens. The diffuse reflectance factor is defined as the ratio of light flux reflected from a sample (e.g., light efflux), to the light flux incident upon the sample (e.g., light influx), and is compared relative to a "perfect white diffuser". The diffuse reflectance measurement excludes specular reflections.
When visually examining a surface critically in order to judge its color, or to discern an image or a pattern, it is not only important to provide illumination to the surface so that the surface is well illuminated, but it is also important that no light is specularly reflected back to the eye. This can be achieved by viewing the plane surface from the normal, and by illuminating the surface 45.degree. to the normal. See, e.g., American National Standard for Photography (Sensitometry), "Density Measurements-Geometric Conditions for Reflection Density", American National Standards Institute )"ANSI") PH 2.17-1985; Commision Internationale l'Eclairage (CIE) No. 15.2 (1986), "Colorimetry."
Accordingly, a need exists for an illuminator for use in colorimetry, spectrophotometry, densitometry or sensitometry which creates the geometric conditions of 45.degree. illumination for viewing or photographing a sample object.
ANSI has established geometric conditions for reflection density or reflectance factor measurements, such as those contemplated for colorimetry, or which conform to the practical situations normally encountered for viewing reflection-type photographs or graphic reproductions.
For general use in colorimetry or spectrophotometry, ANSI has established an applicable standard for illumination, commonly referred to as the 45.degree.:0.degree. (or 0.degree.:45.degree.) standard of illumination. This standard has been specifically selected in order to illuminate an object surface, whose reflection density is to be measured, at print angles between 40 and 50 degrees to the normal of the object surface. The measurements are typically viewed or taken from the normal to the object surface. This standard has been found to produce conditions which tend to minimize surface glare and maximize the density range of the image.
Under the ANSI standard, a 45.degree. cone of light impinges upon the object, while the cone possesses a +/-5.degree. subtended angle. For example, in 45.degree.:0.degree. illumination, the incident light (or "influx") is concentrated in an annulus that impinges at a central 45.degree. angle to the sample, with a subject angular spread of incident light (or subtended angle) between 40.degree. and 50.degree. to the sample. The diffusely reflected light (or "efflux ") is collected normal to the sample. Although the measured efflux is at its maximum detection sensitivity at the normal, the ANSI standard provides that the measured diffuse light possesses a measured subtense of approximately 10.degree. (i.e., a cone of light defined by a surface 5.degree. from the normal).
As described in ANSI PH 2.17-1985, "Density Measurements-Geometric Conditions For Reflection Density", for the 45.degree.:0.degree. standard, the incoming influx radiance is established to have a maximum at 45.degree. to the normal to the sampling aperture, and shall be negligible at angles less than 40.degree. or more than 50.degree. to the normal at any point on the sampling aperture.
Similarly, for 0.degree.:45.degree. illumination, light incident upon the sample surface would be restricted to an influx cone of light having a 0.degree. central angle, but restricted to a cone having an outside surface 5.degree. from the normal. The measured diffused efflux, having a maximum detection sensitivity at 45.degree. from the normal, ranges in a cone restricted to 40.degree. and 50.degree. from the normal.
It will be appreciated by those skilled in the art that, as reflected in the designation of the ANSI standard (45.degree.:0.degree., or 0.degree.:45.degree.), the corresponding advantages attendant with the 45.degree. configuration are obtained whether the object is illuminated at 45.degree. and viewed from the normal, or whether the object is illuminated at the normal and viewed from 45.degree.. Thus, it is understood that the concepts disclosed for the apparatus and method disclosed herein may also be used to provide the efflux geometry for a 0.degree.:45.degree. configuration.
Various illuminators are known in the prior art. For example, U.S. Pat. Nos. 4,022,534 and 4,076,421 to Kishner disclose a 45.degree./0.degree. illuminator/collector geometry. Kishner '534 employs a wedge-shaped diffuser and a non-imaging, cylindrical reflector to obtain 45.degree. illumination, while Kishner '421 employs a diffusing screen with the non-imaging cylindrical reflector to obtain the 45.degree. illumination. The Kishner illuminators, although providing a measure of 45.degree. illumination, suffer in that the illumination at all points along the imaging plane is not uniformly 45.degree.. Uniform 45.degree. is only achieved at the center of the imaging plane due to the non-imaging of the cylindrical reflector. The baffles of Kishner, which can not be telecentric to the cylindrical reflector, are only configured to prevent some passage of non-45.degree. light. Moreover, in Kishner the diffuser does not compensate for variations in the light output of the lamp.
Other illuminators known in the art also suffer from the ability to use available types of lamps, or lamps of wide-ranging shape. For example, U.S. Pat. No. 4,422,135 to McCamy discloses an annular illuminator with a combination of a spheric mirror, an elliptical mirror, and a circular cylindric mirror to create 45.degree. illumination. McCamy requires that the shape of the lamp that is employed must be that nearly approximating a point source of light, and therefore various lamp shapes or lamp types currently in use may not be employed. Further, the illumination angles created are different for points of the object not on the center axis, so that the ANSI 45.degree./0.degree. standard is difficult to meet. Finally, with McCamy a suitable reference for correcting lamp variations is difficult to obtain and, therefore, measurements are subject to drift and poor repeatability.
Accordingly, there exists a need for a conical illuminator which uniformly illuminates an object surface at a uniform 45.degree. angle, at all points on the surface, and which compensates for light output variations, and which can employ a variety of lamp types or lamp shapes. Moreover, there exists a need for a conical receiver optical system that provides uniformity in angular and spacial geometry.